Promote active_admin_form_for into an Arbre component - resolves #2542

Uses FormBuilderProxy as suggested by Igor in #2689, removing and replacing form_buffers from ActiveAdmin::FormBuilder.  As a bonus allows 'inputs', etc. instead of 'f.inputs' in DSL.  Allowing Arbre within has_many block deferred as a second step.  May break existing code that mixes ERB inside active_admin_form_for in a template.  Can also break other code that makes assumptions about the context of the block, see fix to active_admin_comments.rb included.